OpenGL 2.1 is now required to use love.graphics (resolves issue #779.)

Removed love.graphics.isSupported("canvas", "shader", "npot", "subtractive", and "mipmap"), since those features are all guaranteed by the minimum system requirements.

--HG--
branch : minor

src/modules/window/sdl/Window.h

@@ -83,6 +83,8 @@ public:
 
 	const void *getHandle() const;
 
+	void showMessageBox(MessageBoxType type, const char *title, const char *message);
+
 	static love::window::Window *createSingleton();
 	static love::window::Window *getSingleton();